1. Field of the Invention
The invention relates to fabrication of masks suitable for lithographic processes, particularly lithographic processes using an electron beam.
2. Discussion of the Related Art
In device processing, an energy sensitive material, called a resist, is coated onto a substrate, e.g., a silicon wafer which, depending on the state of completion of the device, possibly has overlying regions of semiconductor material, dielectric material, and/or electrically conductive material. Selected portions of the resist are exposed by irradiating the resist though a mask exhibiting a desired pattern, or the inverse of the desired pattern. The pattern is then developed in the resist, generally by immersing the resist in a solvent or by subjecting the resist to a plasma, to remove either the exposed or the unexposed regions. This pattern is then used as a mask, e.g., for etching the desired pattern into the underlying substrate, for performing ion implantation in the desired pattern, or for depositing a material in the desired pattern. Once the resist has served this patterning function, the resist is removed, a layer of material is typically deposited onto the patterned substrate, and the so-called lithographic process is repeated until the device is finished. The desire for ever-increasing miniaturization of devices on integrated circuits has resulted in the evolution of such lithographic processes to provide the requisite level of delineation. While exposure by near-UV rays offers useful results for the current generation of integrated circuits, it is expected that alternative radiation sources will be required for future generations. In this regard, some efforts have focused on the use of x-ray lithography, while others have focused on charged particles, e.g., electron or ion beam delineation.
From these efforts, electron beam lithography appears to be a useful tool for both the present and future fabrication of integrated circuits. However, it was found that conventional stencil or aperture masks, which expose the selected portions of a resist by absorbing portions of the electrons directed at the mask, imposed limitations on electron beam lithographic processes, e.g., due to heating of the absorbing areas, such heating causing mask deformation. An alternative to the conventional stencil or aperture mask was therefore developed, as discussed, for example, in U.S. Pat. Nos. 5,079,112, 5,130,213, 5,260,151, 5,376,505, 5,258,246, and 5,316,879, the disclosures of which are hereby incorporated by reference. The process of these patents, which is known as Scattering with Angular Limitation in Projection Electron-beam Lithography (SCALPEL), relies on a mask that scatters portions of the electrons directed at the mask such that the scattered electrons are capable of being directed at a substrate in a desired pattern. Improvements in the process have also been developed, as reflected, for example, in U.S. Pat. Nos. 5,382,498, and 5,561,008.
The basic features of a mask suitable for use in the process of these patents, or in similar processes based on electron or ion exposure, is shown in FIGS. 1 and 1A, and a schematic of one embodiment of the SCALPEL process is shown in FIG. 2. FIG. 1 shows a mask 10. The mask 10 is optionally supported by a ring 12 that is relatively rigid and heat-tolerant. As shown by the detailed view of FIG. 1A, the mask 10 contains a membrane 14 that is relatively transparent to electrons. The membrane 14 is attached to struts 16 that physically support the membrane 14 and reduce the physical effects of varying thermal and mechanical conditions. Skirts 18 are typically located on the top surface of the membrane 14 overlying the areas supported by the struts 16. (Alternatively, the skirts are located on a bottom surface of a membrane and struts are located on a top surface of the membrane.) The skirts 18 provide characteristics, e.g., substantially straight lines, useful when directing electrons through the mask during a lithographic process. The skirts 18 also prevent the electron beam from impinging on the struts 16, thereby reducing heating of the mask 10, as well as allow some tolerance of errors in placement of the struts 16 and control of the strut profile.
As shown in FIG. 2, the mask 10 is illuminated with electrons 22. Scattering areas 20 are present in a desired pattern on the membrane 14 of the mask 10, and act to scatter the electrons 22 passing through the mask 10. The skirts 18 similarly scatter the electrons 22 passing through the mask 10. (Scattered electrons 34 are also shown in FIG. 2.) Beams of electrons 30, 32 pass through individual segments of the membrane 14 relatively unscattered, the individual segments of the membrane 14 defined by the skirts 18. The unscattered electrons 30, 32 then move through an electromagnetic/electrostatic projector lens system 36, along with the scattered electrons 34. The projector lens system 36 causes the electrons 30, 32, 34 to converge. The optics of the apparatus are designed such that the unscattered electrons 30, 32 will pass through an aperture of a back focal plane filter 37, while the scattered electrons 34 are directed to a nontransparent region of the back focal plane filter 37. The electrons 30, 32 cross over each other as they pass through the aperture 36. Typically, a second projector lens system 38 brings the electrons of each beam 30, 32 into a parallel relationship, and the electron beams 30, 32 then expose the substrate 40 in a desired pattern.
For electron beams 32 that are not aligned with the optical axis of the apparatus, the second projector lens system 38 is also typically designed to redirect the electrons 32 to help compensate for gaps in the transmitted beams of electrons. In particular, as shown by FIG. 2, the presence of the struts 16 creates gaps between incoming groups of electron beams (e.g., beam groups 30, 32). In some situations, it is desirable to eliminate or reduce such a gap prior to the electrons impacting the substrate 40. Thus, as shown in FIG. 2, it is possible to configure the second projector lens system 38 to shift beam groups (e.g., beams 32) to compensate for this gap. In addition, it is possible to use other process parameters, e.g., relative movement of mask 10 and substrate 40, to contribute to this compensation, referred to as stitching. The design and presence of the skirts 18 generally also contributes to the gap compensation.
Clearly the structure of the mask plays an important role in SCALPEL and in similar electron-based or ion-based lithographic processes. The method of forming the mask, however, is exacting and time consuming--particularly forming the strut pattern such that the struts do not detrimentally affect the physical properties of the membrane and do not substantially interfere with the electron transmission, e.g., due to irregularities in shape of the struts. The process for forming masks is expected to become even more difficult as larger masks are developed. Improved methods for forming masks suitable for use in SCALPEL and similar systems are therefore desired.